A siege engine is a device that is designed to break or circumvent city walls and other fortifications in siege warfare.
Ancient siege enginesThe earliest siege engine was the battering ram, followed by the catapult in ancient Greece and the onager invented by the Romans.
Medieval siege engines
Medieval designs include the catapult, the ballista and the trebuchet. These machines used mechanical energy to fling large projectiles to batter down stone walls. Also used were the battering ram and the siege tower, a wooden tower on wheels that allowed attackers to climb up and over castle walls, while protected from enemy arrow fire.
A typical military confrontation in medieval times was for one side to lay siege to their opponent's castle. When properly defended, these high walled fortifications were virtually impenetrable to infantry or cavalry. The attacker then had the choice of simply surrounding the castle and blocking food deliveries and waiting for the defenders to surrender in the face of starvation, or more proactively to employ war machines specifically designed to destroy or circumvent castle defenses.
Other tactics included setting fire to castle walls in an effort to melt the cement that held together the individual stones so they could be readily knocked over. Another indirect means was the practice of sapping, whereby tunnels were dug under the walls to weaken the foundations and destroy them.
Modern siege engines
With the advent of gunpowder, firearms such as the arquebus and cannon—and eventually the mortar and artillery—were developed. These weapons proved so effective as to render the city wall virtually obsolete.